In the medical industry there is widespread use of medical instruments, such as catheters and angioplasty balloons, which are inserted into the patient's body to permit surgical repair, maintenance and examination. For example, catheters generally include a hollow tubular portion, usually formed of resilient plastic, for insertion through the skin of a patient into a cavity, duct or vessel to permit injection or withdrawal of fluids, or to deliver medications to patients for therapeutic reasons. Regardless of the instrument, the accurate placement of the instrument in the patient's body is often critical to its successful use. Moreover, catheters are increasingly being used to introduce a mechanical device into the body through the catheter which, when exiting the catheter, unfolds or springs open into an operating position. Examples of such devices include vena cava filters used to trap blood clots, and stents used to hold blood vessels open. These applications are substantially different than the simple administration of fluids in that the proper function of the mechanical device being passed through the catheter is often even more dependent upon its accurate placement in the patient's body.
As a result, attempts have been made to assist the physician in viewing the catheter during use to assist in the accurate positioning of the device. For example, X-ray reflective chemicals may be used in forming the plastic tubular portion of the catheter to allow the physician to view the catheter using X-ray radiography. Often, however, these specially made catheters remain very difficult to see. For catheters being used to position a mechanical device such as a stent, other methods have been used to improve the visibility of the catheter. For example, to accurately place a stent at a predetermined location in a blood vessel, the physician will first place the tip of the catheter, used to deliver the stent, at that location. The physician knows the catheter tip is correctly placed by injecting a small amount of X-ray reflective solution or "dye" through the catheter, and observing where the dye emerges in the blood vessel. However, as soon as the dye dissipates in the blood flow, the location of the tip of the catheter is no longer as obvious. Regardless, the physician continues with the procedure by pushing the stent (which is plainly visible on X-ray) through the catheter. The physician must watch the stent closely during this time for any sign that it is beginning to open since this is the only way to determine whether the stent has reached the open distal tip of the catheter. However, the physician has no way of knowing if the catheter itself (which still contains the stent) has moved. Therefore, even though the physician knows the stent is about to exit the catheter, it is uncertain whether the stent will be placed exactly in the position desired.
In an attempt to solve the above-noted deficiencies, some catheter manufacturers position a marker device in the form of a radiopaque material near the distal end of the medical instrument. For example, U.S. Pat. No. 5,213,111 discloses a wire guide for a catheter including a coil spring formed of a radiopaque material, such as platinum, positioned around an inner strand which also may be platinum or gold. This design better enables the physician using the wire guide to determine its exact location via X-rays. However, this design is complex and requires a rather large amount of expensive radiopaque material thus increasing the cost of the device. Also, once the catheter has been guided into place and the wire guide removed, it is difficult to monitor the location of the catheter. Various other catheter manufacturers mark the catheter by gluing a marker device in the form of a band of radiopaque material, such as platinum or gold, to the tubular portion near the distal end of the catheter so that the physician can always see the exact location of the catheter. Although these bands are very thin, the outer radial extent of the band still extends beyond the outer radial surface of the catheter creating an annular ridge at each end of the band. These ridges can create difficulties when inserting the catheter into the blood vessel including causing an uncomfortable feeling to the patient. Also, since the band is glued on, it can only be applied to certain materials which permit secure bonding of the glue. Moreover, even then, the band may be detached from the catheter without much difficulty.
Catheters and other medical devices have also been formed of shape-memory alloys and resins, such as a nickel-titanium alloy often called Nitinol. These materials exhibit anthropomorphic qualities of memory and trainability such that when the alloy is heated above a certain transition temperature, a desired shape which has been processed into the material for that temperature is restored regardless of the shape or configuration prior to heating. These materials may be deformed considerably when at a lower temperature but will completely recover to their configuration on being heated above the specified transition temperature. For example, U.S. Pat. Nos. 5,019,040, 5,019,057 and 5,078,684 all disclose catheter devices formed partially of shape-memory alloys which allow the shape of the catheter to be advantageously modified during use. However, these references fail to disclose catheters having simple, yet effective, marker devices which can be easily, inexpensively and yet securely attached to the catheter for enabling the physician to accurately locate the position of the catheter during use relative to the patient.
U.S. Pat. No. 4,198,081 to Harrison et al. discloses a coupling formed of a shape-memory material which uses the shape-recovering properties of the shape-memory material to connect two pipes together. However, the coupling is not used in the medical field as a marker device for a medical instrument. Moreover, this coupling device does not use the shape-recovery properties of the coupling to embed the coupling into the pipes to avoid the formation of an annular ridge at each end of the coupling.